Add Draw.successful()

It can decide if the given draw was successful:

* The word is accepted
* Work was submitted for the word
* The work was submitted on time

words/migrations/0004_work_upload_time.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.utils.timezone
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('words', '0003_draw_word_related'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='work',
+            name='upload_time',
+            field=models.DateTimeField(default=django.utils.timezone.now),
+        ),
+    ]